Method of stabilising a magnetometer signal and stabilised magnetometers

ABSTRACT

The magnetometer includes a magnetic core ( 1 ) with at least one branch ( 2, 3 ), at least one exciting coil ( 4, 5 ) and one take-up coil ( 10 ) sensitive to ambient fields thanks to the excitation field (B). An additional magnetic field, to advantage perpendicular to the previous ones, is added to eliminate very low frequency noises, without it being measured.

[0001] The subject of this invention is a method of stabilising amagnetometer signal and magnetometers so stabilised.

[0002] Magnetometers of the “fluxgate” type considered here include amagnetic core, at least one active coil wound around the core, throughwhich an excitation current passes creating a magnetic field in thecore, and a take-up coil with two windings in series, it too being woundaround the core and fitted with means for measuring the voltage at itsterminals. Most often, the core is a core including two branchesconnected in series, the windings of the take-up coil are placed aroundthe branches respectively, and two active coils are found each of whichincludes two windings placed around the branches respectively. Theactive coils are then wound in the same way around each of the branchesbut the opposite way from one branch to the other, in such a way thattheir effects are strengthened and generate a magnetic field ofcirculation in the core; but the take-up coil is wound in such a waythat the magnetic field produced by the active coils induces countercurrents in its two windings, and which cancel each other out.

[0003] Magnetometers so constructed have the particularity of beinginsensitive to the first degree to the magnetic field produced by theexciting current, but of being sensitive to ambient magnetic fields,which interact with the excitation magnetic field, if the latter is ofsufficient intensity: an induced current, with a frequency double thatof the excitation current and with an intensity proportionate to that ofthe ambient magnetic field, passes through the take-up coil of themagnetometer and can be measured.

[0004] Other, less sophisticated, magnetometers do not includedifferentially connected windings in series of the take-up coil andmeasure an induced current including the excitation frequency and thedouble frequency; the invention may also be applied to them.

[0005] The inventors have however noted that these magnetometers weresubject to not insignificant low-frequency noises, which were able tojam or block out the induced signal representing the magnetic field tobe measured. They have therefore sought to stabilise the total outputsignal by reducing this noise. The solution they propose here consistsin subjecting the take-up coil to an additional alternating magneticfield, to which the measurement means are insensitive.

[0006] This additional magnetic field may have any direction, and can beproduced by the active coil in addition to the magnetic sensitisationfield or by another means.

[0007] The invention will now be described with reference to thefigures:

[0008]FIG. 1 is a general view of the magnetometer,

[0009]FIG. 2 shows a noise obtained without the invention beingimplemented,

[0010]FIG. 3 is a frequency diagram,

[0011]FIGS. 4, 5 and 6 shows three embodiments of the invention.

[0012] The differential magnetometer in FIG. 1 is the most common typeand includes a magnetic core 1 rectangular in shape with two mainbranches 2 and 3 in series. Three coils are wound around the core 1, andmore exactly two active coils 4 and 5 each including two windings 6, 7and 8, 9 around the branches 2 and 3 respectively, and a take-up coil 10composed of two winding 11 and 12 also around the branches 2 and 3respectively. The active coils 4 and 5 are connected in series in acircuit leading to an excitation device 13, and the take-up coil is atthe terminals of a measurement device 14.

[0013] The windings 6 and 8 are formed in the same direction around thebranch 2, and the windings 7 and 9 in an opposite direction around theother branch 3 in such a way as to create a magnetic field B able tocirculate in the magnetic core when it is closed, as is the core 1.

[0014] When the magnetic circuit is open, a magnetic field circulates ineach of the branches and these two magnetic fields arecounter-directional.

[0015] The windings 11 and 12 of the take-up coil 10 are in the samedirection, whereas the windings 6 and 8 have directions counter to thewindings 7 and 9.

[0016] The excitation means are able to supply an excitation current ata pre-set frequency and may include an oscillator, a binary frequencydivider, a square wave to triangular wave converter and a voltage tocurrent converter. The measurement means 14 include a preamplifiercollecting the voltage at the terminals of the take-up coil 10, asynchronous detector supplied by the signal of the oscillator notdivided by two, a low-pass filter, a proportional-integral-differentialcorrector, an amplifier, a low-pass filter associated with the displaymeans and to allow persistence of vision at cut-off frequency of aboutone hertz, and display means. A feedback current may be applied at theterminals of the measurement coil 10 by sampling the voltage at thecorrection module output and converting it into current. No more timewill be spent on these elements, which have already been disclosed andare moreover quite straightforward, and which allow the induced signalto be sampled at a frequency double the excitation frequency and allowit to be converted into a continuous signal prior to transmitting it tothe display means, which measure it.

[0017]FIG. 2 shows that the void signal obtained with a magnetometer ofthis type includes oscillations of an amplitude able to reach about 2microteslas in this example, essentially between two extreme values,with no pronounced periodicity but concentrated at the very lowfrequencies. The sample shown covers a time of 30 seconds. As theamplitude of the signal to be measured is often barely greater than thisnoise, it can be imagined that measurement is very muddled and that animprovement is called for.

[0018] According to the invention, an additional magnetic field isapplied in order to stabilise the measurement. It may be of 350microteslas for a frequency of 60 kHz when the field produced by theexcitation frequency is 0.9 tesla. No other particular requirements havebeen noted in order to obtain an appreciable reduction in very lowfrequency noise, with the result that the additional stabilisation fieldis able to be produced in different ways and have different directions.

[0019] It may be convenient to produce it via an element built into themagnetometer, using already existing means such as the exciting coils 4and 5: the excitation means 13 will then be designed so as to producethe two excitation and stabilisation fields, at different frequenciesand simultaneously.

[0020] In another embodiment of the invention, shown in FIG. 4, anadditional coil 19 is placed around the magnetometer, the two branches 2and 3 and all the other coils previously encountered, and additionalexcitation means 18 are placed at its ends so as to produce theadditional field.

[0021] However if the stabilisation field is parallel to the measurementfield, a drawback is encountered in that the stabilisation field maydazzle or saturate the measurement means due to the overlapping of thefrequency bands.

[0022] Referring to FIG. 3, which is a frequency diagram, the reference15 has been given to the pass band of the ambient signal able then to bedetected, the reference 16 to the excitation pass band and the reference17 to the measurement pass band, in which the induced signal iscollected. These bands 15, 16 and 17 are roughly equidistant, the band15 bringing together the low frequencies and the band 17 being atfrequencies double the band 16.

[0023] It is therefore recommended, if a special means is added tocreate the additional field, that it is placed such that this field isperpendicular to the excitation field. FIG. 5 shows an arrangement ofthis kind: an additional coil 20, supplied by additional excitationmeans 21 at its ends, is orientated in such a way that its axis cuts thebranches 2 and 3 of the core 1, contrary to the arrangement in FIG. 4.

[0024] Instead of an additional single coil like 19 or 20, severaladditional coils could be used, which would be orientated in such a wayas to create an additional field resulting in the chosen direction.Instead of coils, a conductor wire passed through by a current issufficient to create the additional field. A particular embodimentappears in FIG. 6, where two wires 22 and 23 are used, close to thebranches 2 and 3 respectively, and parallel to them such that theadditional field, perpendicular to the wires 22 and 23, is so also tothe excitation field B. In-phase or out-of-phase currents can passthrough the wires 22 and 23; it is therefore possible to unite them in asingle circuit 24 provided with a common excitation means 25.

[0025] The frequency of the additional stabilisation field will toadvantage be lower than the frequency of the band 16 (of the excitationcurrent). However, it may be any size if the additional field isperpendicular to the excitation field or if it has been possible toeliminate completely the signals induced at the excitation frequency inthe coil, via a rigorously symmetrical construction of the branches ofthe magnetometer or by a compensation device; the frequency of theadditional stabilisation field may even be in the measurement band 17.

[0026] But if the additional field is parallel to the excitation field(and to the direction of the ambient field to which the magnetometer issensitive), it will be appropriate to take additional precautions and tochoose the frequency of this additional field outside the detection band15 the excitation band 16 and the measurement band 17.

[0027] If the magnetometer is not differentially connected, it will bepreferable to choose one of the embodiments with the additional magneticfield perpendicular to the excitation field and to the take-up coil.

1. Method of stabilising a magnetometer including a magnetic core (1),coils including at least one active coil (4, 5) and one take-up coil(10), the active coil being arranged in such a way as to create amagnetic excitation field (B) in the core, which sensitises the take-upcoil, means (13) of passing an alternating current at an excitationfrequency in the active coil and means (14) of measuring a voltageinduced at a frequency double the excitation frequency in the take-upcoil, characterised in that it is consists in subjecting the take-upcoil to an additional alternating magnetic field, to which themeasurement means (14) are insensitive.
 2. Method according to claim 1,characterised in that the additional magnetic field is produced by theactive coil.
 3. Method according to claim 1, characterised in that theadditional magnetic field is perpendicular to the take-up coil and tothe excitation field.
 4. Method according to claim 1, 2 or 3,characterised in that the additional magnetic field has a frequencyoutside a band of excitation frequencies (16), a measurement band (17)and a detection band (15).
 5. Magnetometer suitable for implementing amethod in accordance with any one of claims 1, 3, or 4, characterised inthat it includes a coil (19, 20) surrounding the magnetic core (1) so asto produce the additional magnetic field.
 6. Magnetometer suitable forimplementing a method in accordance with any one of claims 1, 3, or 4,characterised in that it includes at least one conductor wire (22, 23)passed through by a current so as to produce the additional magneticfield.
 7. Magnetometer according to claim 6, characterised in that itincludes as many conductor wires as the core (1) includes branches (2,3), the wires being respectively close to the branches and parallel tothe branches.